1. Field of the Invention
The present invention relates to an inverter apparatus configured to execute instantaneous current control and thereby to drive motors including three-phase alternating-current motors (such as an induction motor or a permanent magnet synchronous motor) and to a semiconductor device and the like used for the same.
2. Description of the Related Art
Today, inverter apparatuses configured to execute instantaneous current control are widely used in refrigerating and air-conditioning machines, air blowers, washing machines, compressor drivers, and the like. Vector control is widely applied to control of, particularly, alternating-current motors such as induction motors or permanent magnet synchronous motors. Put simply, the vector control is a control method by means of detecting a motor current and performing special calculation to separate the motor current into a magnetic flux component and a torque component. When changing a motor output, for example, it is possible to achieve efficient control by controlling only a torque current.
Hence, in order to perform the vector control, it is necessary to detect alternating current that flow in each phase of a motor in principle. Three-phase induction motors and permanent magnet synchronous motors configured to flow three-phase alternating currents are widely used in this regard. FIG. 1 shows an example of a detection circuit that uses shunt resistors for detecting currents on a permanent magnet synchronous motor (hereinafter also abbreviated as PMSM) Although hall elements have been often used in detection circuits in the past, shunt resistors are frequently used today. This is because the shunt register has features including a smaller external shape and a more excellent temperature characteristic than the hall element.
As shown in FIG. 1, a method of detecting currents by using three shunt resistors 131, 132, and 133 is called a three-shunt detection method. The method will be briefly described as follows. For example, in the case of current detection of a U-phase, a voltage between both ends of the shunt resistor 131 are amplified with an operational amplifier 134, while a current Iu that is AD-converted with an AD (analog to digital) converter 151 included in a microprocessor (hereinafter also referred to as a microcomputer) 150 is sent to a motor controller 160. The publicly-known vector control is performed here. Based on a result of the control, a PWM (pulse-width modulation) generator 152 outputs a signal for controlling a switching element, and thereby the switching element is controlled. Current flow to the motor is eventually controlled. In the meantime, a method of detecting current values of two phases out of three phases by using a relation of Iu+Iv+Iw=0 is called a two-shunt detection method. Further, there is also proposed a single-shunt detection method which is configured to estimate currents of each phase by detecting a direct-current value using a single shunt resistor instead of using multiple shunt resistors and thereby to perform the vector control.
However, due to its nature, the single-shunt detection method faces many practical problems in order to achieve efficient control with a simple configuration. One of the problems is that the method requires high performance and high speed for microprocessor for controlling the inverter and the like.
The three-shunt detection method is the best among these methods of performing the vector control using the shunt resistors from the viewpoint of ease of control. However, the three-shunt detection method requires current detection at high accuracy. Therefore, it is essential to avoid deviations in resistance values among the three shunt resistors, variation in the resistance values with time, and so forth. In this context, Japanese Patent Application Publication No. 2004-225580 (hereinafter referred to as Patent Document 1) discloses a technique for reducing deviations in electrical resistance values attributable to variation in welding operations for attaching shunt resistors to an inverter-integrated electric compressor, for example. Moreover, Japanese Patent Application Publication No. 2003-235268 (hereinafter referred to as Patent Document 2) discloses a technique for correcting resistance values of shunt resistors by executing temperature measurement in order to prevent deviations in temperature characteristics of the shunt resistors in the case of a rise in temperature of a motor.
Meanwhile, although a voltage between both ends of each of the shunt resistors are amplified by use of the operational amplifier 134, 135 or 136 as shown in FIG. 1 in many cases, an offset attributable to its temperature characteristic may be caused in the operational amplifier. An accurate current value cannot be obtained with an offset caused in an operational amplifier even if a shunt resistor detects an accurate potential difference in each phase. Based on this concept, Japanese Patent Application Publication No. 2003-324985 (hereinafter referred to as Patent Document 3) discloses a motor control device which is capable of canceling an influence of temperature variation, detecting a current accurately, and thereby achieving appropriate motor control.
As described above, substantial efforts have heretofore been devoted to improvement in accuracy of resistance measuring means including the shunt resistor and the operational amplifier. In other words, tremendous efforts have been devoted to pursuit of higher accuracy of a current detector. Nevertheless, this attempt contradicts market demands such as cost reduction.
Meanwhile, as opposed to the pursuit of higher accuracy of the current detector that has been the conventional problem-solution approach for avoiding deviations in resistance values among shunt resistors, Japanese Patent Application Publication No. 2006-230766 (hereinafter referred to as Patent Document 4) discloses a washing machine motor capable of preventing occurrence of output torque fluctuation as much as possible even when a current detector for detecting a current flowing on each phase of an inverter has poor detection accuracy. FIGS. 2A and 2B show a method disclosed in Patent Document 4. In the method, to describe it shortly, a voltage is applied to each coil of the three phases at the time of product shipment or service in the market, and then an average value of currents flowing at that moment is measured. Based on the average value, a correction coefficient is found, and thereby resistors are corrected. At the time of normal use, a current correction value is found based on the value of the correction that is given, by use of the correction coefficient.